We often hear how the rent in San Francisco is some of the priciest in the country, but there are also just not enough places for people to live.

The mayor’s office has asked city officials to speed up the lengthy permitting process in efforts to increase the housing supply. But some say that the supply already exists -- that thousands of units are simply being kept off the market. Some estimate up to 10,000 of these units exist. Many sit unrented because tenants are proving too risky an investment for some property owners.